1. Field of the Invention
The invention pertains to methods and apparatus for the production of cement clinker.
2. The Prior Art
With the introduction of raw meal pre-heaters into the cement clinker burning process, rotary kilns which up to then had been long could be built shorter. With modern burning systems incorporating conventional cyclone pre-heaters, a ratio between the length of the rotary kiln to the inside rotary kiln diameter of approximately 15:1 through 17:1 has become general in practice. Efforts of the system manufacturers to further reduce the relative rotary kiln length have failed because of the safety requirements of the system users. Longer rotary kilns have tended to result in disruption-free operation, particularly with respect to interior cycles.
With the introduction of pre-calcination technology with secondary firings in the pre-heating system, a high degree of deacidification of the raw meal was achieved before entry into the rotary kiln. The advantages of lower heat introduction in the sintering zone of the rotary kiln, however, were only partially exploited. The kiln diameter was approximately reduced in relationship to the proportionate heat amounts supplied, on the one hand, to the pre-burner locations and, on the other hand, to the sintering zone of the rotary kiln. Until now, the standard ratio of kiln length to inside kiln diameter (15:1 through 17:1) mentioned above was approximately retained. As a result, specific rotary kiln lengths which were still large ensued, which resulted in high investment and operating costs. On the other hand, a reduction of the diameter of the rotary kiln, given the same throughput capacity and the same degree of oven charge, produced a shorter dwell period of the raw meal or, respectively, clinker in the rotary kiln. Therefore, until now, a further shortening of the rotary kiln was not considered.
The substances volatilizing in the rotary kiln often exhibit harmful components such as alkali compounds and sulfur which, upon their condensation from the rotary kiln exhaust gas, lead to caking in the lines conducting the gas. Moreover, these harmful components enter the raw meal pre-heating system together with the rotary kiln exhaust gas where they precipitate on the raw meal and are reintroduced into the rotary kiln with the pre-heated raw meal. A highly accumulating cycle of harmful substances can thus be formed in the burning process. In order to avoid this disadvantage, it is conventional to draw off a portion of the hot rotary kiln exhaust gases from the burning process via a bypass and to discard it. The significant heat content of the thermally highly valuable exhaust gas of the rotary kiln is, however, lost from the burning process by this technique. As a result, the entire burning process can become uneconomical. Particularly in very large cement manufacturing systems, it can no longer be justified in terms of heat efficiency to discard too much rotary kiln exhaust gas containing harmful substances without exploiting its heat content. By so doing, the manufacturing costs of cement, given today's energy costs and those to be expected in future, become too high.